There is an increasing public interest in chemoprevention by natural agents such as n-3 polyunsaturated fatty acids (PUFAs) DHA and EPA and the pregnancy-induced differentiation against breast cancer incidence. The mechanisms underlying these preventive effects are currently unknown. Little is known about the regional and developmental expression of locally acting factors in the mammary epithelium that interact with n-3 PUFA and exert differentiating effect during pregnancy. Within this content, a novel mammary derived growth inhibitor and a fatty acid binding protein (FABP) has recently been identified, characterized, and named Mammary derived growth inhibitor Related Gene (MRG). As a new member in the family of FABP, MRG has the highest binding affinity to n-3 PUFA DHA. The preliminary data indicate: (1) MRG overexpression suppresses breast cancer cell growth in vitro and tumorigenesis in vivo. (2) MRG induces differentiation of mammary epithelium and its expression is associated with human mammary gland differentiation. (3) MRG synergistically interacts with DHA in growth inhibition. The present application is to test the hypotheses that (a) MRG is a mediator for intracellular accumulation of n-3 PUFAs in mammary epithelial cells and mediates tumor suppressing effect of n-3 PUFAs on mammary tumors. In this regard, one of mechanisms for pregnancy-induced prevention may be mediated in part by PUFA composition change in mammary gland which favors the ratio of n-3 PUFAs to n-6 PUFAs following pregnancy and lactation; (b) MRG is mediator of the differentiating effects of pregnancy on breast epithelium and overexpression of MRG in young nulliparous mice can induce differentiation and protect the development of mammary tumors. We propose a series of studies of both in vitro and in MMTV/MRG transgenic mice to evaluate the role of MRG on mammary differentiation and tumorigenesis and its interaction with n-3 PUFA on mammary tumorigenesis. These studies will help to identify MRG as mediator for differentiation for mammary gland and for interaction with n-3 PUPA on inhibition of mammary tumorigenesis.